One of the most difficult problems in in vivo imaging of living organisms is how to distinguish between normal and aberrant tissue. Many approaches to this problem have been developed, including inter alia, X-ray imaging (including CAT-scanning), radionuclide imaging, fluoroscopy, ultrasonic imaging and nuclear magnetic resonance (NMR) imaging (MRI), with and without the administration of imaging agents, e.g., contrast media. The imaging agent may comprise materials which are themselves opaque to the detection signal and simply increase the contrast between organs or tissues containing it and organs or tissues which do not, e.g., as with X-ray agents. Alternatively, the agent can be one which has a local effect on the endogenous moiety active to the modality, as in the effect of NMR contrast agents on protons in vivo. For example, such agents may comprise materials which are selectively biodistributed due to pharmacokinetics or affinity for a certain compound, cell type, tissue, organ etc. In the latter case, the agent will highlight those areas containing the matter for which the agent, e.g., based on an antibody e.g., a monoclonal antibody, has affinity, e.g., a cell-surface antigen. In the former, it will highlight the areas where it is selectively transported. Many such imaging agents are well known in the relevant arts, as are methods of use thereof.
A number of methods have been explored for enhancing tumor uptake of monoclonal antibodies.
One approach is the use of interferons to augment tumor antigen expression. J. W. Greiner et al., "Augmentation of Tumor Antigen Expression by Recombinant Human Interferons: Enhance Targeting of Monoclonal Antibodies to Carcinomas," Cancer Imaging with Radiolabeled Antibodies, D. M. Goldenberg ed., Kluwer Academic Publishers, Boston, pp. 413-432 (1990). One problem with this approach is that the effect is limited to certain antigens and thus cannot be applied to a broad spectrum of tumors and antibodies. This method does not increase the accessibility of the antibody to the tumor, but by increasing available binding sites may increase the target-to-non-target ratio.
Anti-antibodies have been used to clear out circulating radiolabeled antibody and hence improve the target-to-non-target ratios of radioisotope in the tumor compared to the surrounding normal tissue. R. M. Sharkey et al., "Anti-Antibody Enhancement of Tumor Imaging," Cancer Imaging with Radiolabeled Antibodies, D. M. Goldenberg ed., Kluwer Academic Publishers, Boston, pp. 433-455 (1990). An alternative to this approach is the use of exchange diffusion to remove unbound radiolabeled antibody from the circulation. C. Henry et al., "Improved Monoclonal Antibody Tumor/Background Ratios with Exchange Transfusions," Antibody Immunoconjugates Radiopharm. 4, 22 (1991). Both methods function by decreasing the background, and hence do not enhance uptake of the radiolabeled antibody in the tumors.
Vasodilators have been conjugated to antibodies to enhance their uptake. B. LeBerthon et al, "The Development of a Novel Vasoactive Immunoconjugate to Enhance the Uptake of Monoclonal Antibodies in Tumors," Antibody Immunoconjugates Radiopharm. 4, 42 (1991). These investigators coupled human IL-2 to antibodies and demonstrated a 3-fold increase in tumor uptake. The problem with this method is that it requires the production of a complex molecule. External beam radiation focused on tumors has been used by S. E. Order et al. to enhance tumor uptake or radiolabeled antibodies administered for radiotherapy of tumors. J. S. Msirikale et al., "Radiation Enhancement of Radiolabeled Antibody Depositions in Tumors," Int. J. Radiat. Oncol. Biol. Phys. 13(12), 1839-44 (1987). The external beam radiation presumably causes increased vascular permeability in the tumor thereby enhancing uptake of the radiolabeled antibody. This study used 131-I labeled anti-ferritin for targeting hepatoma, preceded by external radiation.
Tumor necrosis factor TNF-.alpha. has been used in conjunction with a monoclonal antibody immunoconjugate and shown to enhance tumor uptake, while IL-1 and IL-2 used in conjunction with a monoclonal antibody immunoconjugate does not enhance antibody targeting. G. A. Pietersz et al., "The Use of Immunoconjugates in Conjunction with Biological Response Modifiers," Antibody Immunoconjugates Radiopharm. 4, 205 (1991).
One major difference between the use of biological response modifiers to enhance antibody targeting to tumors and biomodulators is that biological response modifiers produce a more generalized or non-specific effect, while the biomodulators preferentially effect change in aberrant or abnormal tissue. The biomodulators may induce the secretion of biological response modifiers in situ so that the effect is more localized than that obtained by systemic administration of a biological response modifier. Thus, one hypothesis is that biomodulators induce biologic response modifiers within tumors causing a localized reaction which results in enhanced tumor uptake of a radiolabeled antibody in the circulation during the induced response. Because the action is localized, biomodulators are less toxic than biologic response modifiers.
Each of the known agents and methods suffers from a variety of deficiencies related to tolerability of the imaging agent, invasive nature of the active radiation and efficiency and accuracy of the diagnosis enabled by the resulting image. For example, NMR imaging is the most safe in terms of the radiation used. It does not involve ionizing radiation as do X-ray and radiodiagnostics.
Under many circumstances each modality provides very detailed information by imaging of various tissues. However, each suffers from a limitation based upon the lack of distinction between normal and aberrant tissue which has the same signature. Several approaches have been taken toward increasing the specificity of contrast agents (often in combination with targeting agents, e.g., antibodies and small biomolecules), thereby expanding the applicability of a given modality. However, even such improvements are insufficient, e.g., suffering from insufficient tissue specificity (target tissue-to-non-target tissue ratios), insufficient target tissue uptake on an absolute basis, insufficiently rapid pharmacokinetics of tissue uptake, etc.
What is needed is a contrast agent which is of increased specificity for aberrant tissue (e.g., tumors) versus its normal tissue counterpart, and/or of increased and/or more rapid aberrant tissue uptake, etc. Antibody-based drug delivery suffers from analogous defects as described for the imaging area above and analogous improvements are needed.